I've had many a stuck bolt freed in my time just by using a propane torch and heat the case around it and then use a chissel on the out side flange of the bolt
not the bolt head itself , don't hit or mar the case just the flange of the bolt
( I think they call them cap head screws or bolts ) drain plugs always have this type of plug in them... the hex head and a flange for the "O"ring do your pounding on the flange at an angle and it will come out of there.
I use a chisel to start the place where I want to pound against then a punch...
or a flat punch ....
several times I have almost removed the entire flange by beating it away
before it finally came free... keep an eye on it if you can get it to move your almost there..... if it moves 1/4" then stops you may need to start another place in the flange so you can pound on as it turns
.... there is an art to Hammer and chisel work believe it or not, not maring the mating surface is possible....but hard to do !
think of it as your using a 1" lever to try and turn the bolt and hit it accordingly at that angle
....
I have a selection of different chisels and punches, bent punches and all
I bet you could use them now !
hang in there, you'll get it !
clean living and perseverance will prevail !LOL....
Bob.......